Sulphur dioxide generators are placed in cartons containing table grapes, the gaseous sulphur dioxide in the carton preventing the growth on the grapes of certain forms of fungi. Where the grapes are to be shipped over long distances it is conventional to use two stage generators. These generate sulphur dioxide relatively rapidly when first placed in the carton. The fast rate of release lasts for a few days and thereafter sulphur dioxide is released at a much slower rate over a period of several weeks. In the most commonly used two stage generator, a paper substrate is coated with a layer comprising a binder in which sodium metabisulphate is dispersed. Sodium metabisulphate, in the presence of moisture, releases sulphur dioxide. Sulphur dioxide from said layer constitutes the first stage release. The generator is further formed with closed pockets in which granular metabisulphate is contained. Water vapour only penetrates the material which forms the pockets after a few days, whereupon the slow, long term second stage release commences.
Much research work has been done to determine the optimum concentration of sulphur dioxide in the atmosphere of the carton. This research work has not yet produced results that have been accepted by the industry as conclusive but it does appear as if much lower sulphur dioxide concentrations than were previously considered desirable are adequate to prevent fungi growth on the grapes. This also tends to lower the sulphite level in the grapes and to reduce any sulphur dioxide related damage.
The present invention seeks to provide a simple, inexpensive sulphur dioxide generator which will maintain an acceptable sulphur dioxide concentration in a carton for a period of weeks during shipping or for months if the grapes are stored for later out-of-season sale.